1. Field of the Invention
The present invention relates to launch vehicles. In particular, the present invention relates to multi-stage launch vehicles which are capable of launching manned or unmanned payloads into Earth orbit.
2. Description of Prior Art and Related Information
The various approaches to launch vehicle design may be generally classified into single stage or multi-stage launch vehicle systems. Single stage launch vehicles employ a single thruster stage which includes all the propellant required to deliver a specified velocity to the payload. Since considerable mass is contained in the propellant tanks, engines and thrust structure, which mass becomes unnecessary once propellant therein is expended, a single stage launch vehicle is inherently of less than optimum efficiency. Multi-stage launch vehicles, where an entire stage, including propellant tanks and engines, is jettisoned after propellant expenditure, have accordingly been developed and gained predominance for earth orbit launch applications.
A Titan IV vehicle, 2 illustrated in FIG. 1 is an example of the current multi-stage launch vehicle art. It can be flown as a three or four stage launch vehicle for space missions in Low Earth Orbit (LEO), or Geosynchronous (GEO) and Geosynchronous Transfer (GTO) Orbits. The first stage consists of two uprated solid rocket motor (SRMU) boosters 4 and 6 "strapped-on" to the second and third liquid propellant stages, 8 and 10. The three stages operate in series, i.e. the thrust of each stage is initiated after previous thrusting stages have expended their propellants and been staged. For GEO missions, the fourth stage 12 is a modified Centaur stage propelled by two LO.sub.2 /LH.sub.2 (liquid oxygen/liquid hydrogen) cryogenic engines.
The Titan IV vehicle 2, will fail to perform its mission successfully if either of two categories of failures occur in the vehicle; 1) a catastrophic failure which destroys the vehicle, or 2) a benign failure, which does not destroy the vehicle, but causes its performance to be degraded so that it does not deliver the required velocity to the payload to place it in its mission orbit.
The Shuttle Launch Vehicle 14 illustrated in FIG. 2, is another example of the current art. The Shuttle is a two stage vehicle with two solid rocket boosters (SRBs), 16 and 18, comprising the first stage. A second stage is propelled by a propulsion module 20 comprised of three space systems main engines (SSMEs), mounted in an orbiter vehicle 21, burning liquid propellants (LO.sub.2 /LH.sub.2) from a single External Tank 22 (ET). Should an SSME fail benignly, i.e., not in a manner which would destroy the vehicle, the Shuttle is designed to enter an abort mode which has a high probability of saving its payload and the orbiter 21 and its crew, but a low probability of placing the payload in its mission orbit. Thus, current multi-stage launch vehicles are limited to having an engine-out capability in a single liquid stage to provide an enhanced probability of preserving a high value payload, and the manned space craft and its crew.
The reason that the application of an engine-out capability in a liquid stage has been limited to the case of the Shuttle is largely economic, i.e., in order that a vehicle perform its mission successfully in spite of loss in performance due to a benign engine failure, it must be launched with a lower payload weight than for the case of all engines thrusting. Thus, the economics of placing payloads into space orbits have mitigated against the use of engine-out capabilities except in the extraordinary case of reducing the probability of loss of the high value payload and the orbiter and its crew.
A detailed review of the in-flight history of U.S. space launch vehicles reveals that most of the failures have been due to inadequate thrust of their propulsion systems. Furthermore, it can be shown that the preponderance of the failures which caused the reduction in thrust were benign in the sense that they didn't cause the destruction of the launch vehicle but caused a loss in velocity delivered to the vehicle thereby failing to deliver the payload to the prescribed mission orbit. Most often this resulted in the payload receiving a velocity less than that required to place it in Earth orbit causing its return to Earth with destruction on impact. Accordingly, there presently exists a need to improve launch vehicle mission failure probability (reliability), payload performance, and economics in spite of benign failures in the vehicle.